To analyze the time evolution of the phase difference between two dynamical elements, the difference of the PCF is considered [1]}.
For Eq. (REF ), it is defined as \(\Gamma _d(\psi )={\Gamma }_1(-\psi )-{\Gamma }_2(\psi )\) .
Then, the phase difference \(\psi =\theta _{1}-\theta _{2}\)  obeys
\(\psi (t+\Delta t)\approx \psi (t)+\Delta t\Gamma _d(\psi (t)).\) 
